Ultrasound imaging has a wide application in the field of medical practice. Ultrasonic diagnostics refers to the imaging of a region of the human or animal patient using an ultrasound transducer to generate and receive ultrasound waves. Typically, the transducer is placed on the patient's body over the region to be imaged and high frequency sound waves are generated by the transducer and directed at the region. The transducer receives reflected ultrasonic waves from the region and converts the received waves into electrical signals from which an image is generated. Due to the extremely high acoustic reflectivity of gases, contrast agents comprised of gas bubbles with and without encapsulating shells are used to improve the quality of ultrasound images by highlighting the blood pool and the vascular perfusion of organs within the body.
The use of ultrasound contrast agents serving also as drug carriers has been described for gas-filled liposomes in U.S. Pat. No. 5,580,575. A quantity of liposomes containing drug is administered into the circulatory system of a patient and monitored using ultrasonic energy at diagnostic levels until the presence of the liposomes are detected in the region of interest. Ultrasonic energy is then applied to the region that is sufficient to rupture the liposomes to release drugs locally for therapeutic purposes. The ultrasonic energy is described in U.S. Pat. No. 5,558,082 to be applied by a transducer that simultaneously applies diagnostic and therapeutic ultrasonic waves from therapeutic transducer elements located centrally to the diagnostic transducer elements.
The use of gas-filled microcapsules to control the delivery of drugs to a region of the body has also been described in U.S. Pat. No. 5,190,766 in which the acoustic resonance frequency of the drug carrier is measured in the region in which the drug is to be released and then the region is irradiated with the appropriate sound wave to control the release of drug. Separate ultrasound transducers are described for the imaging and triggering of drug release in the target region.
Exemplary contrast agents include, for example, stabilized microbubbles, sonicated albumin, gas-filled microspheres, gas-filled liposomes, and gas-forming emulsions. A variety of methods have been developed for their manufacture. These methods usually involve spray drying, emulsion, or interfacial polymerization techniques. Typically, the result is a microbubble population having a range of diameters with either a fixed or an arbitrarily variable wall thickness. An ultrasonic contrast agent produced by one methodology, for example, may contain microbubbles where each has a shell wall of the same thickness regardless of its diameter. Alternatively, a different method of production may result in a microbubble population with wall thickness varying even between those microbubbles having the same diameter.
Conceptually, for an ultrasound contrast agent to be used as a carrier for therapeutics, the agent would typically be, through processing, internally loaded with a drug. The treated microbubbles are then injected intravenously and allowed to circulate systemically. An ultrasound signal of sufficient energy to rupture the drug-containing microbubbles is applied to a region where the delivery of the drug is desired. The insonating beam destroys the microbubbles and thus releases its payload.
An ultrasound contrast agent having a fixed or an arbitrarily variable wall thickness may not by optimal as a carrier of therapeutic agent. A microbubble population having an arbitrary wall thickness could result in the drug being released prematurely or not at all. Those with thinner more fragile walls may rupture from hydrostatic pressure before reaching the site. Those with thicker more durable walls may not rupture at all. A microbubble population with a fixed wall thickness would similarly be unsuitable. While the strength of an encapsulated microbubble is a function of the thickness of its wall, it is also a function of its diameter. Thus, a relatively smaller microbubble would show more resistance to hydrostatic and acoustic pressures than would a relatively larger bubble having the same wall thickness.
A drug-containing ultrasound contrast agent having a controlled fragility would therefore represent an improvement to the state of the art. For purposes herein, the term “controlled fragility” is taken to describe a microbubble population having the characteristic of being rupturable only when exposed to acoustic energy equal to or greater than a predetermined intensity. That is, below this acoustic intensity threshold, substantially all the microbubbles remain intact while above the acoustic intensity threshold the microbubbles rupture. While in the unruptured state, bubble agents can still be seen ultrasonically in the larger blood pool so that the sonographer can position and focus the scanner transducer on the region of interest prior to increasing ultrasound intensity to initiate agent rupture and concomitant delivery of drug. Thus, the agent can be turned-on or turned-off by controlling the intensity of the insonating signal.